Description is made, with reference to FIG. 1, about a structural example of a laser diode driving circuit (LD driving circuit) which uses a boost circuit to act as a supply source of a driving current of a laser diode (LD).
The LD driving circuit shown in FIG. 1 has a boost circuit 11 and an optical output automatic power control circuit (APC circuit) 12.
A supply voltage Vcc is boosted by the boost circuit 11 and applied to the collector of a transistor Tr1 connected to the laser diode LD. In the APC circuit 12, an optical output from the laser diode LD is photoelectrically converted by a photodiode PD, and a monitoring current passing through a resister R3 is input to an inverting amplifier circuit as a monitoring voltage. Then, an output from the inverting amplifier circuit is input to the base of a transistor Tr3. The collector of the transistor Tr3 is connected to the transistors Tr1 and Tr2 that make up a current mirror circuit, and a driving current corresponding to a base potential of the transistor Tr3 passes through the laser diode LD. When the amount of luminescence of the laser diode LD is reduced due to fluctuations in temperature or the like, the monitoring current of the photodiode PD is reduced. Then, the output from the inverting amplifier circuit is increased and the base potential of the transistor Tr3 is increased, and thereby the driving current of the laser diode LD is increased. Reversely, when the amount of luminescence of the laser diode LD is increased, the APC circuit 12 operates to reduce the driving current of the laser diode LD. Thus, the optical output from the laser diode LD is controlled automatically.
A related-art example of the LD driving circuit is described in the patent literature 1. The LD driving circuit of the patent literature 1 detects an optical output of a semiconductor laser by a photodiode and generates a laser driving current based on a monitoring voltage of the photodiode. A transmitter including a boost circuit is described in the patent literature 2. The boost circuit applies a boost voltage to a light-emission driving part including an infrared-emitting diode. Additionally, the patent literature 3 discloses that a photodiode detects a partial optical output from a semiconductor laser diode and a laser driving circuit controls a driving current of the semiconductor laser diode based on its detection results.